The present invention pertains to a process for gas phase removal of metals from substrates such as semiconductor devices and micromechanical devices.
The removal of trace metallic contamination is an important step in the semiconductor fabrication process. Trace metals can cause degradation of device performance if not removed from the wafer surface. Metallic contamination can arise from a number of sources including wet chemicals, photoresist, ion implantation and redeposition of sputtered materials from chamber surfaces during plasma processing.
Copper, in particular, is known to deposit from the hydrofluoric acid solutions commonly used to strip oxides in semiconductor fabrication. Aluminum alloyed with a small percentage of copper is commonly used to fabricate connecting metal lines in device fabrication. Copper is also present in small quantities in 304 and 316 stainless steels, common chamber and piping fabrication materials.
Particularly because of its tendency to deposit from HF solutions and therefore remain on the surface in an "HF last" cleaning sequence, copper is one of the most difficult transition metals to remove by traditional wet cleaning techniques.
Many different etching and cleaning techniques have been developed for various semiconductor processes. Typically in the past, wet etches, such as "RCA Clean", dominated in semiconductor fabrication processes. Gradually, as device structures have shrunk and the move toward VLSI devices grown, dry etches have gained prominence. These dry etches include plasma and gas-based etches and many were developed originally for removing oxides and carbon-based contaminants.
For dry gas-phase metal removal, several systems have been reported. U.S. Pat No. 5,094,701 and U.S. Pat. No. 5,221,366 disclose use of beta-diketone and beta-ketoimine ligand forming compounds, which are dispersed in an oxidizing atmosphere. At a sufficient temperature, volatile metal-ligand complexes are reported to be formed and then sublimed from the surface. Temperatures of 200.degree.-300.degree. C. are indicated to be required. U.S. Pat. No. 5,213,621, U.S. Pat. No. 5,213,622, and U.S. Pat. No. 5,332,444 disclose other ligand forming chemical reagents which reportedly can be used in a similar manner to form volatile metal-ligand complexes with surface impurities which then can be sublimed from the surface.
Other dry gas-phase removal techniques involve the use of ultraviolet irradiation to generate cleaning radicals. Sugino et al. (IEIC Trans. Electron. Vol. E75-C, No. 7, July 1992) describe a system for removal of Fe and Al on a silicon surface using photoexcited chlorine radicals at approximately 20 Torr and 170.degree. C. as a cleaning gas. U.S. Pat. No. 5,221,423 to Sugino discloses a method for removing Al, Fe, Na and Cr by irradiating chlorine gas at a partial chlorine pressure of 20 Torr to produce chlorine radicals. U.S. Pat. No. 5,178,721 to Sugino discloses a UV radical generating cleaning method in which the pressure of the chlorine gas and the pathlength of the UV are varied to maximize radical generation and cleaning efficiency. In that system, the chlorine pressure ranges from 1 Torr to atmospheric pressure. Ito (Proc. Instit. for Environ. Studies 1991 p.808) discloses a method for cleaning using photoexcited chlorine radicals wherein the chlorine is delivered at a pressure of 20 Torr. Unfortunately, techniques that rely on the generation of chlorine radicals are harsh on the substrate and on the cleaning apparatus because the chlorine radicals etch the substrate and cleaning apparatus in addition to the impurities.
There continues to be a need for improved processes for removal of trace metals from semiconductor substrates and substrates in micromechanical devices and the like, for processes that are not harsh on the substrate or processing equipment.